Apply a Function on a List
Run a JavaScript function on every list item. Runs entirely in your browser, so your data never leaves your device.
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Output
The result appears here as you type.
How to use Apply a Function on a List
- 1. Paste your list. Enter your items into the input pane, one per line. Any list you want transformed by custom logic rather than a fixed built-in operation works here.
- 2. Write a function. Type a JavaScript function body in the Function (item, index) => result field. It runs once per item, receiving the item's text and its position, and returns the transformed value.
- 3. Copy the transformed list. The output pane shows one line per item, each replaced by the result your function returned. Copy the transformed list wherever the new values are needed.
When to use Apply a Function on a List
Apply a Function on a List runs a small JavaScript function against every item in a pasted list, letting you write custom logic instead of relying on a fixed built-in transformation. Use it whenever a specific list tool doesn't already exist for your case.
- Uppercasing every item with a one-line function. You need every item converted to uppercase, and writing 'item.toUpperCase()' as the function body applies that transformation across the whole list in one pass.
- Adding the item's position into the text. A list needs each item annotated with its own index, such as 'apple (0)', which a function using both the item and index parameters can produce directly.
- Applying a custom calculation to a list of numbers. A list of numeric strings needs each value doubled or converted with a formula, and writing that calculation as a function applies it to every line at once.
- Combining several transformations in one pass. A list needs trimming, case conversion and a prefix added all at once, and writing one function that does all three avoids running three separate tools in sequence.
Examples
Uppercase every item
Input
cats dogs
Output
CATS DOGS
Number the items
Input
cats dogs
Output
1. cats 2. dogs
About the Apply a Function on a List tool
Apply a Function on a List is a free online tool that works entirely inside your web browser. Run a JavaScript function on every list item. Because the processing happens on your own device, nothing you enter is uploaded, logged or stored anywhere.
This page is one of 114 List utilities on EditSafely. Each one does a single job well, and all of them follow the same rule: your input stays on your machine.
You can shape the output with the Function (item, index) => result setting, and the result refreshes the moment you change it. 2 worked examples further down the page show exactly what the tool produces for real inputs.
Because nothing leaves your device, the tool is suitable for sensitive content such as internal documents, credentials or customer data. It also responds instantly, since every keystroke is handled on your own machine rather than by a remote API.
Frequently asked questions
Is Apply a Function on a List free to use?
Yes, it is completely free. All 2,658 tools on EditSafely work without an account, a subscription or usage limits.
Is it safe to paste sensitive or confidential data?
Everything happens locally. Your browser downloads the tool's code once, then does all the processing itself; nothing you enter is transmitted, stored or logged. You can even go offline after the page loads and it will still work.
How much text can I process at once?
There is no fixed limit. Because the work happens on your own device rather than on a shared server, the practical ceiling is your machine's memory, which comfortably handles inputs far larger than typical online tools allow.
Do I need to sign up or install anything?
No. The tool works in any modern browser on desktop, tablet or phone. There is no account to create, no extension to add and no software to install.
How do I use the result?
The output panel has a one-click copy button, and you can keep refining the input while you work; the result updates in place as you type.